In 1994, AC-3 marketed as Dolby Digital® was accepted by the ATSC as the audio compression system for the new American terrestrial broadcast digital television standard. At that time, DIRECTV® was already delivering digital transmission to the United States via satellite. For audio compression, DIRECTV® was broadcasting using “MPEG level 1” audio compression providing stereo audio. Dolby Digital® AC-3 won the ATSC selection committee by providing for slightly better compression as well as means of handling a wide array of programming modes up to “5.1 channel”. 5.1 channels of surround sound provides for five distinct full fidelity channels, representing: right front, center front, left front, right rear and left rear channels, plus one limited bandwidth “Low Frequency Enhancement” channel. This selection of channels matches what has been available for presentation at movie theaters. The technical details for Dolby Digital® AC-3 is well described as part of the ATSC standard in the ATSC document A/52. This document, as well as the entire ATSC specifications, is available on the World Wide Web at www.atsc.org.
A satellite broadcaster provides multiple channels of recently released movies available for viewing on a Pay-Per-View (PPV) basis. This service competes with the VHS tape rentals market and companies. A competitive edge may be provided by the combination of convenience and quality.
Dolby Digital® with 5.1 channels surround sound has come available on DVD releases. Tape marketers would have a quality advantage for the home theater segment of this market unless technology could be developed to permit broadcasters to transmit such audio features. In the fall of 1997, DIRECTV® undertook the project to broadcast full 5.1 channels of audio into the homes of their customers. On Jul. 1, 1998 DIRECTV® began regular commercial broadcast of Dolby Digital 5.1 channel surround sound, begin the first broadcaster to provide such a service.
The prior practice for handling audio within a broadcast environment is as follows: Audio starts at the source as either analog audio, or digital audio in a generally uncompressed format. The audio is mixed to a final “release” version and then possibly lightly compressed for delivery to the broadcast facility. At that broadcast facility, the audio would again be brought down to an uncompressed format and at the last step in the broadcast chain be fed to a real time audio compression. This compression step would do the final “heavy” lossy audio compression for transmissions to the integrated receiver decoders (IRD) used by the end customers.
In this project DIRECTV® was first to bring Dolby Digital® that was encoded at the movie studio by broadcasting that audio “studio direct” to the customer. This required the development of specific applications in the art to meet this objective. These developments are not obvious from the existing AC-3 technology itself, and many obstacles had to be overcome to develop “studio direct” broadcasting of this multiple channel audio standard. Specifically, Dolby Digital® contains what is called as “meta data”, that being ancillary data that is used to control the decoder process. This “meta data” routinely changes on a scene by scene basis, depending on plot of the movie. Examples of “meta data” present in a Dolby Digital® data stream are discussed below.
An LFE is a bit which enables the low frequency enhancement channel. Much of the time this is turned off, providing extra bandwidth availability for the main audio channels. It is enabled where the director wishes to “shake the house”. A Dialogue Normalization is a value that defines the dynamic range of the audio with respect to the normal dialog level. Mix Level is an information quantity regarding how to mix a 5.1 channel presentation down to a stereo mix. A Surround Sound Mix Level is a control for the down mix (that reduces the number of channels finally output) levels of the surround sound channels for reproduction as stereo or Dolby Pro-Logic outputs. A Compression gain meta tag controls the decoder dynamic range when the end customer selects a mode of operation that provides a narrow dynamic range.
To do a proper job of encoding Dolby Digital® AC-3, all the above meta data must be supplied correctly by someone knowledgeable of the content. The person most qualified to do provide this information is the sound engineer responsible for mixing the movie at the studio. The ability to deliver to the end customer exactly the same compressed data as created by the sound engineer is a very desirable feature, but not readily available for AC-3 multiple channel audio with the previous broadcast technology.